1. Field of the Invention
This invention relates generally to temperature control of a plant having plural temperature control zones. In particular, this invention relates to controlling devices affecting temperature in temperature control zones wherein control in one or more selected zones is effected according to the effective control parameter of another zone.
2. Description of Related Art
It is known to arrange controls for devices affecting temperature of controlled plants, whether the controlled plant is equipment or a building, in plural temperature control zones. Each temperature control zone associates at least one device for affecting temperature with a control for controlling operation of the device. The number of zones and the proximity of devices from zone to zone are chosen to achieve an acceptable range of temperature variation within the temperature control zones. For example, in temperature control of injection molding equipment, it is known to provide plural temperature control zones to maintain the so-called melt at a desired temperature as it is conveyed from within the injection unit where raw material is plasticized to article defining cavities of a mold assembly. More particularly, in such equipment comprising a so-called hot runner system for conveying melt from an injection unit to mold cavities, it is known to provide plural temperature control zones within the hot runner system.
In known temperature controlled plants, it is known to provide control for a zone wherein temperature measured by a sensor associated with a zone is compared with a desired temperature for the zone to produce a control signal for devices affecting temperature within the zone. It is also known to provide temperature control for plants wherein control of devices in one or more selected zones where temperature measurements are not available is “slaved” to an effective control parameter for a temperature affecting device in a “master” zone where a temperature measurement is available. The underlying premise for such master/slave control arrangements is that elements affecting temperature of the master and slave temperature control zone(s) be sufficiently similar so that temperature in slave zones can be controlled with acceptable precision in the absence of direct temperature measurement. It is contemplated that in addition to designed master/slave arrangements chosen to reduce the number of temperature sensors used, circumstances may arise wherein measured temperature information ceases to be available for particular zones, necessitating operation in master/slave arrangements until availability of temperature measurement information is restored. Loss of temperature measurement information may arise from failure of a sensor, failure of communication between the sensor and control, or failure of control components that process sensor signals to generate measured temperature information for subsequent control processing.
It is known that because of differences in temperature control characteristics between slave zones and the master zone, use of the effective control parameter of the master zone can result in the temperature of the slave zone(s) deviating from the desired temperature. While such deviations are of no consequence in some controlled plants, they may have adverse consequences in others. A hot runner system for injection molding is an example of a controlled plant in which such deviations may have adverse consequences, potentially resulting in production of articles of unacceptable quality. Differences in temperature may arise from structural differences between the master and slave temperature control zones as well as from dynamic differences associated with operation of the plant. Examples of structural differences include differences in relative locations of heat sources within temperature control zones, differences in dimensions and/or fit-up of mechanical elements affecting heat transfer, and differences in electrical properties of devices controlled to affect temperature. Examples of dynamic differences include differences in properties of the applied primary source of energy (heat) controlled to affect temperature, and localized environmental differences affecting heat transfer, such as differences in local ambient temperature proximate the master and slave temperature control zones. Hence, a need exists to accommodate such differences to insure desired temperature control of slave zones in master/slave arrangements for temperature control.